Systemic immune activation in early postnatal life is capable of altering the reactivity of the hypothalamic-pituitary-adrenal (HPA) axis to stress and behavioral responsiveness to social interaction in later life. We propose two experiments to address the possibility that early social interactions between infants and mothers are key contributors in early immune-activation effects on adult social behaviors. Using a well-defined animal model of selectively bred mice in a cross-fostering experimental design, we assess the contributions of individual differences in infant responsiveness to immune stimuli, and the contributions of changes in maternal behavior to the prediction of adult outcomes. Immune activation will be manipulated by i.p. injection of endotoxin or saline to neonates. In Study l, treated neonates will be cross-fostered into different maternal caregiving environments. In Study 2, infants with apparent line-differences in sensitivity to endotoxin are crossed into similar caregiving environments. The findings generated are expected to direct next steps in the search for the determinants of the developmental effects of this ubiquitous and non-obvious environmental influence on behavior. Understanding the process responsible for this developmental effect may contribute to a more general account of how organisms receive influences from the environment and how these stimuli are fused with the process of maturation to produce competent individuals.